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Real-Time Learning with Hadoop's Lambda Architecture
- 1. 1 ©MapR Technologies -‐ Confiden6al Real-‐&me Learning with Hadoop The “λ + ε” architecture
- 2. 2 ©MapR Technologies -‐ Confiden6al § Contact: – tdunning@maprtech.com – @ted_dunning § Slides and such (available late tonight): – hEp://slideshare.net/tdunning § Hash tags: #mapr #storm
- 3. 3 ©MapR Technologies -‐ Confiden6al The Challenge § Hadoop is great of processing vats of data – But sucks for real-‐6me (by design!) § Storm is great for real-‐6me processing – But lacks any way to deal with batch processing § It sounds like there isn’t a solu6on – Neither fashionable solu6on handles everything
- 4. 4 ©MapR Technologies -‐ Confiden6al This is not a problem. It’s an opportunity!
- 5. 5 ©MapR Technologies -‐ Confiden6al t now Hadoop is Not Very Real-‐&me Unprocessed Data Fully processed Latest full period Hadoop job takes this long for this data
- 6. 6 ©MapR Technologies -‐ Confiden6al t now Hadoop works great back here Storm works here Real-‐&me and Long-‐&me together Blended view Blended view Blended View
- 7. 7 ©MapR Technologies -‐ Confiden6al An Example
- 8. 8 ©MapR Technologies -‐ Confiden6al The Same Problem
- 9. 9 ©MapR Technologies -‐ Confiden6al What Does the Lambda Architecture Do? § The idea is that we want to compute a func6on of a all history up to 6me n § In order to get real-‐6me response, we divide this into two parts § Where the addi6on may not really be addi6on § The idea is that if we lose the history from m+1 un6l n, things get beEer soon enough f x1...xn( ) f x1...xm...xn( )= f x1...xm( )+ f xm+1...xn( )
- 10. 10 ©MapR Technologies -‐ Confiden6al Can We Do BeLer? § Can we minimize or avoid failure transients? § Can we guarantee precise boundaries? § Can we synchronize computa6ons accurately?
- 11. 11 ©MapR Technologies -‐ Confiden6al Alterna&ve without Lambda Search Engine NoSql de Jour Consumer Real-‐6me Long-‐6me ?
- 12. 12 ©MapR Technologies -‐ Confiden6al Problems § Simply dumping into noSql engine doesn’t quite work § Insert rate is limited § No load isola6on – Big retrospec6ve jobs kill real-‐6me § Low scan performance – Hbase preEy good, but not stellar § Difficult to set boundaries – where does real-‐6me end and long-‐6me begin?
- 13. 13 ©MapR Technologies -‐ Confiden6al Almost a Solu&on § Lambda architecture talks about func6on of long-‐6me state – Real-‐6me approximate accelerator adjusts previous result to current state § Sounds good, but … – How does the real-‐6me accelerator combine with long-‐6me? – What algorithms can do this? – How can we avoid gaps and overlaps and other errors? § Needs more work § We need a “λ + ε” architecture !
- 14. 14 ©MapR Technologies -‐ Confiden6al A Simple Example § Let’s start with the simplest case … coun6ng § Coun6ng = addi6on – Addi6on is associa6ve – Addi6on is on-‐line – We can generalize these results to all associa6ve, on-‐line func6ons – But let’s start simple
- 15. 15 ©MapR Technologies -‐ Confiden6al Data Sources Catcher Cluster Rough Design – Data Flow Catcher Cluster Query Event Spout Logger Bolt Counter Bolt Raw Logs Logger Bolt Semi Agg Hadoop Aggregator Snap Long agg ProtoSpout Counter Bolt Logger Bolt Data Sources
- 16. 16 ©MapR Technologies -‐ Confiden6al Closer Look – Catcher Protocol Data Sources Catcher Cluster Catcher Cluster Data Sources The data sources and catchers communicate with a very simple protocol. Hello() => list of catchers Log(topic,message) => (OK|FAIL, redirect-‐to-‐catcher)
- 17. 17 ©MapR Technologies -‐ Confiden6al Closer Look – Catcher Queues Catcher Cluster Catcher Cluster The catchers forward log requests to the correct catcher and return that host in the reply to allow the client to avoid the extra hop. Each topic file is appended by exactly one catcher. Topic files are kept in shared file storage. Topic File Topic File
- 18. 18 ©MapR Technologies -‐ Confiden6al Closer Look – ProtoSpout The ProtoSpout tails the topic files, parses log records into tuples and injects them into the Storm topology. Last fully acked posi6on stored in shared, transac6onally correct file system. Topic File Topic File ProtoSpout
- 19. 19 ©MapR Technologies -‐ Confiden6al Closer Look – Counter Bolt § Cri6cal design goals: – fast ack for all tuples – fast restart of counter § Ack happens when tuple hits the replay log (10’s of milliseconds, group commit) § Restart involves replaying semi-‐agg’s + replay log (very fast) § Replay log only lasts un6l next semi-‐aggregate goes out Counter Bolt Replay Log Semi-‐ aggregated records Incoming records Real-‐6me Long-‐6me
- 20. 20 ©MapR Technologies -‐ Confiden6al A Frozen Moment in Time § Snapshot defines the dividing line § All data in the snap is long-‐6me, all awer is real-‐6me § Semi-‐agg strategy allows clean combina6on of both kinds of data § Data synchronized snap not needed (if the snap is really a snap) Semi Agg Hadoop Aggregator Snap Long agg
- 21. 21 ©MapR Technologies -‐ Confiden6al Guarantees § Counter output volume is small-‐ish – the greater of k tuples per 100K inputs or k tuple/s – 1 tuple/s/label/bolt for this exercise § Persistence layer must provide guarantees – distributed against node failure – must have either readable flush or closed-‐append § HDFS is distributed, but provides no guarantees and strange seman6cs § MapRfs is distributed, provides all necessary guarantees
- 22. 22 ©MapR Technologies -‐ Confiden6al Presenta&on Layer § Presenta6on must – read recent output of Logger bolt – read relevant output of Hadoop jobs – combine semi-‐aggregated records § User will see – counts that increment within 0-‐2 s of events – seamless and accurate meld of short and long-‐term data
- 23. 23 ©MapR Technologies -‐ Confiden6al The Basic Idea § Online algorithms generally have rela6vely small state (like coun6ng) § Online algorithms generally have a simple update (like coun6ng) § If we can do this with coun6ng, we can do it with all kinds of algorithms
- 24. 24 ©MapR Technologies -‐ Confiden6al Summary – Part 1 § Semi-‐agg strategy + snapshots allows correct real-‐6me counts – because addi6on is on-‐line and associa6ve § Other on-‐line associa6ve opera6ons include: – k-‐means clustering (see Dan Filimon’s talk at 16.) – count dis6nct (see hyper-‐log-‐log counters from streamlib or kmv from Brickhouse) – top-‐k values – top-‐k (count(*)) (see streamlib) – contextual Bayesian bandits (see part 2 of this talk)
- 25. 25 ©MapR Technologies -‐ Confiden6al Example 2 – AB tes&ng in real-‐&me § I have 15 versions of my landing page § Each visitor is assigned to a version – Which version? § A conversion or sale or whatever can happen – How long to wait? § Some versions of the landing page are horrible – Don’t want to give them traffic
- 26. 26 ©MapR Technologies -‐ Confiden6al A Quick Diversion § You see a coin – What is the probability of heads? – Could it be larger or smaller than that? § I flip the coin and while it is in the air ask again § I catch the coin and ask again § I look at the coin (and you don’t) and ask again § Why does the answer change? – And did it ever have a single value?
- 27. 27 ©MapR Technologies -‐ Confiden6al A Philosophical Conclusion § Probability as expressed by humans is subjec6ve and depends on informa6on and experience
- 28. 28 ©MapR Technologies -‐ Confiden6al A Prac&cal Applica&on
- 29. 29 ©MapR Technologies -‐ Confiden6al I Dunno 0 0.2 0.4 0.6 0.8 1 p Prob(p)
- 30. 30 ©MapR Technologies -‐ Confiden6al 5 heads out of 10 throws 0 0.2 0.4 0.6 0.8 1 p Prob(p)
- 31. 31 ©MapR Technologies -‐ Confiden6al 2 heads out of 12 throws 0 0.2 0.4 0.6 0.8 1 p Prob(p) Mean Using any single number as a “best” es6mate denies the uncertain nature of a distribu6on Adding confidence bounds s6ll loses most of the informa6on in the distribu6on and prevents good modeling of the tails
- 32. 32 ©MapR Technologies -‐ Confiden6al Bayesian Bandit § Compute distribu6ons based on data § Sample p1 and p2 from these distribu6ons § Put a coin in bandit 1 if p1 > p2 § Else, put the coin in bandit 2
- 33. 33 ©MapR Technologies -‐ Confiden6al And it works! 11000 100 200 300 400 500 600 700 800 900 1000 0.12 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 n regret ε-greedy, ε = 0.05 Bayesian Bandit with Gamma-Normal
- 34. 34 ©MapR Technologies -‐ Confiden6al Video Demo
- 35. 35 ©MapR Technologies -‐ Confiden6al The Code § Select an alterna6ve § Select and learn § But we already know how to count! n = dim(k)[1]! p0 = rep(0, length.out=n)! for (i in 1:n) {! p0[i] = rbeta(1, k[i,2]+1, k[i,1]+1)! }! return (which(p0 == max(p0)))! for (z in 1:steps) {! i = select(k)! j = test(i)! k[i,j] = k[i,j]+1! }! return (k)!
- 36. 36 ©MapR Technologies -‐ Confiden6al The Basic Idea § We can encode a distribu6on by sampling § Sampling allows unifica6on of explora6on and exploita6on § Can be extended to more general response models § Note that learning here = coun6ng = on-‐line algorithm
- 37. 37 ©MapR Technologies -‐ Confiden6al Generalized Banditry § Suppose we have an infinite number of bandits – suppose they are each labeled by two real numbers x and y in [0,1] – also that expected payoff is a parameterized func6on of x and y – now assume a distribu6on for θ that we can learn online § Selec6on works by sampling θ, then compu6ng f § Learning works by propaga6ng updates back to θ – If f is linear, this is very easy § Don’t just have to have two labels, could have labels and context E z[ ]= f (x, y |θ)
- 38. 38 ©MapR Technologies -‐ Confiden6al Caveats § Original Bayesian Bandit only requires real-‐6me § Generalized Bandit may require access to long history for learning – Pseudo online learning may be easier than true online § Bandit variables can include content, 6me of day, day of week § Context variables can include user id, user features § Bandit × context variables provide the real power
- 39. 39 ©MapR Technologies -‐ Confiden6al § Contact: – tdunning@maprtech.com – @ted_dunning § Slides and such (just don’t believe the metrics): – hEp://slideshare.net/tdunning § Hash tags: #mapr #storm
- 40. 40 ©MapR Technologies -‐ Confiden6al Thank You